1. Field of the Invention
The present invention relates to a vehicle steering system employing an SBW (steering by wire) system, in which an operation section (or controls) operated by a driver and a steering section (or mechanism) for steering steered wheels are not mechanically coupled with each other.
Priority is claimed on Japanese Patent Application No. 2005-162394, filed Jun. 2, 2005, the content of which is incorporated herein by reference.
2. Description of the Related Art
A vehicle steering system employing the SBW system has (i) a steering actuator for steering the steered wheels in accordance with actions of the driver who operates a steering wheel (i.e., a control), and (ii) a reaction actuator for providing appropriate reaction (i.e., a reactive force) for steering (i.e., steering reaction) when the driver operates the steering wheel, where the two actuators are independently controlled.
In a known method for controlling steering reaction, the reactive force for steering is generated based on a difference between a target steering angle, which is set based on operation of the steering wheel, and an actual steering angle of the wheels (see, for example, Japanese Unexamined Patent Application, First Publication No. 2004-210024). Generally, the control is performed in a manner such that the greater the difference between the target steering angle and the actual steering angle, the greater the generated reactive force is.
However, in the above conventional steering reaction control based on the above difference, the steering reaction is influenced by steering-follow-up performance which depends on dynamic characteristics of the actual steering mechanism; thus, the driver may feel uncomfortable with respect to the steering reaction.
For example, generally, in steering while the vehicle speed is very low, the output from the steering mechanism is low and the steering-follow-up performance tends to be lower. In this case, the difference between the target steering angle and the actual steering angle increases, so that a larger reactive force is generated, which may make steering feeling worse.
In addition, a large output from the steering mechanism is required when the steering mechanism has a large inertia. In such a case, when a high-speed steering operation using the steering wheel is performed, the difference between the target steering angle and the actual steering angle is large in an initial stage, and then the difference attenuates. Therefore, in the initial stage of steering, much reactive force is generated and the steering operation feels very heavy for the driver, and after that, the driver feels the termination of the reaction; therefore, the steering operation feels uncomfortable for the driver.
On the other hand, the steering system may have an assistance system for the steering operation of the driver. In an example of an assistance system, in order to prevent the action of the vehicle from becoming disordered due to disturbance such as a crosswind, an independent or an additional steering operation, not based on the driver's intention, is performed at the system side, so as to stabilize the action of the vehicle. Such a steering system is called “active steering” below.
However, in the conventional SBW system employing active steering, the difference between the target steering angle and the actual steering angle is influenced by the active steering, so that the reactive force includes a component from the active steering, thereby making the steering feeling worse.